During 1980-1994, the number of twin births in the United
States increased by 42%, from 68,339 to 97,064, and the twin birth
rate (i.e., the number of twin births to total live births)
increased 30%, from 18.9 to 24.6 per 1000 live births. These
increases are important because the risks for preterm birth, low
birthweight (LBW), long-term disability, and early death are
greater for twins than for singletons (1; CDC, unpublished data,
1991). To estimate state-specific rates of twin births, CDC
analyzed data from the U.S. certificates of birth for 1992-1994.
This report presents the findings of this analysis of these data,
which indicate that state-specific rates of twin births varied
substantially, and the variations reflect factors other than
state-specific differences in maternal age distributions.

In this analysis, twin births were defined as individual live
births in twin deliveries, rather than sets of twins (e.g., a
delivery resulting in one live birth and one stillbirth is reported
as one birth from a twin delivery). Because the type of twin (i.e.,
monozygotic {resulting from the fertilization of one ovum} or
dizygotic {resulting from the fertilization of two ova}) is not
listed on birth certificates, this analysis could not distinguish
between twin types. To improve the reliability of the
state-specific estimates of rates of twin births, data from
1992-1994
were combined. Because rates of twin births increased with
increasing maternal age, state-specific rates were standardized to
the U.S. maternal age distribution for 1992-1994 to account for
differing age distributions.

During 1992-1994, the rate of twin births in the United States
was 24.0 per 1000 live births. Among the 50 states and the District
of Columbia, rates ranged from 19.8 (Idaho and New Mexico) to 27.7
(Connecticut and Massachusetts). Rates were highest for the New
England, Middle Atlantic, and East North Central regions
(Table_1),
(Figure_1). The 10 highest rates were reported for Connecticut,
Massachusetts, New Jersey, Rhode Island, Illinois, Michigan, New
York, Delaware, Ohio, and Maryland. In general, rates for states in
the South and West were substantially lower than the overall rate
for the United States; in particular, six of the 10 states in the
Mountain region (New Mexico, Idaho, Utah, Montana, Arizona, and
Wyoming) accounted for the lowest rates.

In general, in states with rates of twin births higher than
the overall rate for the United States, the maternal age
distribution was older than that for the United States overall.
Consequently, rates in these states generally decreased after
standardization (Table_1). However, rates for nine of the 10
states
with the highest observed rates remained significantly higher than
the U.S. rate even after standardization. For five of these states
(Connecticut, Massachusetts, Illinois, Michigan, and Ohio), rates
ranked among the 10 highest after standardization. The persistent
differences between rates for these states and the overall rate for
the United States primarily reflected higher rates among mothers
aged greater than or equal to 25 years (Figure_2); age-specific
rates for states with the highest rates generally were similar to
U.S. rates for mothers aged less than 25 years but higher for
mothers aged greater than or equal to 25 years.

In general, in states with rates of twin births lower than the
overall rate for the United States, the maternal age distribution
was younger than that for the United States overall. Consequently,
rates in these states generally decreased after standardization
(Table_1); however, rates for nine of the 10 states with the
lowest
observed rates remained significantly lower than the U.S. rate even
after standardization. The 12 states with the lowest observed rates
also had the lowest adjusted rates, although the rank order changed
slightly.

The state-specific variation in rates of twin births also
reflected state-specific differences in racial/ethnic composition,
although in some states the small numbers of twin births for which
detailed age and racial/ethnic information was listed precluded
reliable standardization (2; CDC, unpublished data, 1994). For
1994, the twin birth rate among non-Hispanic white mothers was
24.3; among non-Hispanic black mothers, 28.3; and among Hispanic
mothers, 18.6. However, accounting for these differences does not
completely account for state variation in twin births. For example,
even after simultaneously adjusting for maternal age, race, and
Hispanic origin, rates of twin births for Connecticut and
Massachusetts remained significantly higher than the rate for the
United States overall.

Editorial Note

Editorial Note: The findings in this report document substantial
state-specific variation in rates of twin births for 1992-1994;
however, this variation is accounted for only in part by
state-specific differences in maternal age distributions. State
variation in rates of twin births also can be influenced by
differences in the use of therapies that enhance fertility (e.g.,
fertility drugs and techniques). These therapies have been
associated with the recent increase in multiple births (3-5).
Although reliable estimates of state-specific use of fertility
drugs are not available, use of in vitro fertilization (IVF) varies
widely by state (6). In addition, during 1992-1994, a total of 11
states had mandated insurance benefits for fertility treatment,
including IVF (7). Differences among states in mandated benefits
also may influence state rates of twin births.

Although twin births constitute only approximately 2% of all
births, the risk for LBW among twins is seven times greater than
that among singletons. In addition, twins account for 17% of all
LBW infants and approximately 12% of all infant deaths.
Consequently, state rates of LBW are at least partially influenced
by the rate of twin births in the state. For example, if rates of
twin births were equal to that of the total United States, the LBW
rate in Connecticut would be 5% lower than the observed rate and
the New Mexico rate, 2% higher. State-specific rates of twin births
also may influence other indicators of infant health, such as state
infant mortality rates.

The findings in this report can be used by public health
agencies, health-care organizations and researchers, and
policymakers in evaluating and planning programs related to infant
health. The sustained increase in the proportion of multiple births
in the United States requires continued assessment to clarify the
influence of state-specific multiple-birth rates in state
comparisons of infant health indicators.

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